Integrated circuits are often provided with debug circuitry which allows the integrated circuit to be debugged. The integrated circuit usually comprises a bus and a plurality of modules connected to the bus which put packets onto the bus. The debug circuitry is one of these modules. The modules also usually include a CPU. In order to operate, the debug circuitry is arranged to receive information from an external tool, put that information onto the bus and to check the response to that information or to output the response to the external tool. The debug circuitry can also carry out internal checks within the integrated circuit.